Bengal silts link Himalayas’ birth to Asian monsoon

An Indian
research team has discovered clues to geological upheavals such as the rise of
the Himalayas and its link to the onset of Asian monsoon in the sediments of
Bay of Bengal.

Monsoons eroded the Himalayas and blew dusts forming silts that
rivers transported to the sea, building the seafloor sediments. By studying the
rate and pattern of sediment deposition in the Bay of Bengal over millions of
years, scientists from CSIR-National Institute of Oceanography (NIO) and Oil
and Natural Gas Corporation Limited have found that the deposition increased in
two phases — from 65 to 54 and 34 to 23 million years ago1.

About 23 million years ago, the Ganges and Brahmaputra
rivers added huge volumes of sediments to the sea at varying rates, ranging
from 40 to 1000 metre per million years. A distinct increase in sediment deposition
during this period provides an important age marker corresponding to rapid rise
of the Himalayas.

Lead scientist Kolluru Sree Krishna
“Stratigraphic records show abrupt changes in
sediment deposition rates and architecture of the seafloor sediments about 23
million years ago,” says Kolluru Sree Krishna from NIO, Goa. This led the
scientists to infer that geological phenomena such as the rapid uplift of the Himalayas,
intensified erosion and the start of rapid sedimentation in the Bay of Bengal
were strongly related to each other, Krishna told Nature India.

The Indian
plate collided with the Asian plate about 40 million years ago. This collision
closed the Tethys Sea and gave birth to the Himalayas, the highest mountain
range, bringing about dramatic changes in regional and global climates. Previous
studies had failed to link the sediment deposition in the Bay of Bengal to the
rise of the Himalayas and the onset of Asian monsoon.

To identify
the link between these geological phenomena, the scientists imaged more than 10
km thick sediments in the Bay of Bengal. Using air guns, they generated seismic
waves that propagated through the entire sediment column and reflected back
from the various geological boundaries to the sea surface. The sea-surface receivers
recorded the reflected signals which were then used to develop images of the
entire sediment column.

Images of the
sediment column reveal that pathways and deposition rates in the Bay of Bengal greatly
varied in space and time, says Krishna. Peninsular rivers such as Cauvery,
Krishna, Godavari and Mahanadi discharged sediments into sea at a rate of 20
metre per million years, with the thickness of sediments reducing from margin
to deep water before 65 and between 56 and 34 million years ago. During these
times, the Bay of Bengal also received sediments from rivers such as Subarnarekha,
Damodar, Ganges and Brahmaputra at an average of 70 metre per million years.

The
massive volcanic eruptions that formed the Deccan Trap flood basalt might have
affected the course of river systems about 65 million years ago, influencing
the rates of sediment discharge into the Bay of Bengal, Krishna points out.

Sediment
deposition rate increased to 210 metre per million years between 6.8 and 0.8
million years ago. This rate further increased from 0.8 million years to
present, reaching from 290 metre to 1600 metre per million
years. “The increased sediment
deposition rates may generally indicate that monsoon intensified in the Asian
region,” Krishna says.

Ocean records are one of the main data sources to understand the complex interactions between active solid mountains and changing climate because these records are nearly continuous and sensitive, says Subhajit Sinha from the Department of Geology, University of Calcutta.

This study, Sinha says, indicate that sediment accumulation in the Bay of Bengal is clearly related to the pulsating tectonic activity and climate change on the highlands of the Himalayas. Besides, it throws light on temporal change in sedimentation patterns – the type of paleochannels and the quantity of sediments they discharged into the sea over millions of years, providing information about long-term weathering and erosion processes, he adds.